Aeroelastic Study and Analysis of Turbomachinery Components

Aeroelasticity is the branch of physics and engineering that studies the interactions between the inertial, elastic, and aerodynamic forces that occur when an elastic body is exposed to a fluid flow. The primary objective of aeroelastic study and analysis of turbomachinery components is to improve the design of turbine engine rotors and safety and efficiency. Modern aeroelastic analysis requires a thorough understanding and application of advanced computational methods such as the nonlinear frequency domain harmonic balance method and advanced computer codes such as NASA’s TURBO flow solver for modeling complex turbomachinery configurations.

Turbomachinery blade rows are designed to contain identical blades within a given blade row. However, minor differences in shape, structural properties and material properties are inherently present among the blades in all bladed disks. These variations arise because of manufacturing tolerances, wear and tear, and other similar factors. The variation in blade properties and the flow field from one blade to another is referred to as mistuning. Mistuning coupled with flexibility of the disk can localize the vibration energy of the system to a small sector of the disk containing only a few blades. These blades could experience vibratory stresses several times higher than other blades not influenced by mistuning. High vibratory stresses result in early blade failures due to fatigue. Current design methods assume a rotor to be tuned (all blades identical) and do not quantitatively account for the effects of mistuning on the fatigue life of the rotor. A typical mistuned pattern of blade vibration is shown in the Figure below for an example rotor.

Variations in incoming flow, blade shape, and blade seating of the disk give rise to aerodynamic mistuning. These differences result in pressure force variations from one blade to another. This also has an impact on the nature of the interaction and coupling between the blades and the disk. This disk-blade coupling is the main cause for localization of vibratory stresses. Current unsteady aerodynamic modeling methods for such effects are prohibitively expensive to account for blade-to-blade variations in aerodynamic properties.

N&R Engineering has upgraded the TURBO code to enable the modeling of a 2-blade-row counter-rotating open rotor fans for aircraft engines, and it includes the capability to compute operating and performance parameters for open rotor fans including advance ratio, power coefficient, torque coefficient and efficiency. Also, it is compatible with the unsteady analysis capability for vibrating blades that is currently available in the TURBO code.

N&R Engineering also has incorporated the Harmonic Balance (HB) nonlinear frequency domain solution method into the TURBO flow solver code. With the HB capability, one can now perform computations for a single blade passage, versus having to model all of the blade passages on a rotor disk as is typically required for time domain computational approaches. This results in a significant computational cost savings of the HB method as compared to conventional time domain solution methods.

N&R Engineering is also using the HB method within the TURBO flow solver code to efficiently model inlet flow distortions, again, with only one blade row passage being necessary to model. This capability allows for a wide range of possible inlet distortions to be rapidly modeled in order to study aeroelastic stability.

In addition to the HB method for modeling inlet flow distortions, N&R Engineering also extended the TURBO code to perform fully coupled inlet-fan interactions that accurately simulate the flow distortions through an inlet duct due to the ingestion of boundary layer flow. This capability provides the foundation to compute fan aerodynamic and aeroelastic responses to the inlet distortion as it propagates throughout the fan.

The detection of possible fan failure issues early in the fan design process can help prevent the high costs associated with engine failure in actual flight operation. The capabilities described above will be very useful for future blended wing body aircraft designs. These designs may incorporate engine inlets integrated into the body of the aircraft to take advantage of efficiency improvement due to weight and drag reduction.